Conventionally, as a secondary battery, which is a kind of a power storage device, a lithium-ion secondary battery and a nickel-hydrogen secondary battery or the like are well-known, for example. In a power storage device manufacturing process, there are a step of housing an electrode assembly inside a case of a power storage device and then sealing the case, and a step of injecting an electrolyte to the case. Further, sometimes there is a step of inspecting airtightness of the case before injecting the electrolyte. For example, Patent Literature 1 discloses an airtightness inspection of detecting presence/absence of gas leakage while forcibly feeding a gas into a housing.
In addition, when injecting an electrolyte, for example, Patent Literatures 2-5 disclose a technology of reducing a pressure inside a container before liquid injection and injecting the liquid by utilizing a differential pressure between an inner side and outer side of the container. Generally, an amount of the electrolyte to be injected into a battery case is a fixed amount (prescribed amount) determined beforehand, and the amount is not changed for each battery case. As a technology of accurately verifying an amount of the injected electrolyte, as described in Patent Literatures 2, 3 and 6 for example, a technology of measuring weight of a battery case before and after the injection is known.
However, since a cavity inside a battery case in which an electrode assembly is housed is different for each battery strictly, it is preferable to inject an appropriate amount of the electrolyte to each battery. Therefore, in a manufacturing process of a power storage device using the electrolyte such as a lithium-ion battery, it has been examined to measure a capacity of an internal space of a case after sealing the case of the power storage device and before injecting the electrolyte into the case. For example, in a lithium-ion battery, when the electrolyte in a liquid state not impregnated in an electrode assembly is remained inside the case, waste of the electrolyte is generated. In addition, when the electrolyte becomes further excessive, it contributes to gas generation. On the other hand, when the amount of the electrolyte is insufficient, a part not sufficiently impregnated with the electrolyte is generated in an active material layer of laminated positive and negative electrodes and a separator (hereinafter, called an electrode assembly), and a performance of a battery is lowered. Therefore, it is desirable to inject an amount of the electrolyte which is optimum for each battery. For variation in terms of manufacturing of the case and the electrode assembly, as one of measures to obtain a liquid injection amount which is optimum for each battery, measurement of the capacity of the internal space is conceivable. For example, Patent Literature 4 discloses a cavity volume measuring device which obtains a cavity volume inside a battery can using Boyle's law.